The emergence of new consumer markets in the personal communications field has driven manufacturers to develop smaller, lighter, more reliable, and cheaper products. These products generally combine RF, microwave, analog, and digital components to form a single unit such as a personal communications handset, for example a cellular phone handset. In a cellular phone handset there are RF circuits which include various passive and active components, such as (on the receive side) diplexer filter, low noise amplifiers, image reject filters, mixers, oscillators, IF amplifiers and various other components. On the RF transmit side, it includes power amplifiers and combiner circuits, in addition to the low frequency digital circuits. The required RF filters, especially the diplexer and image filters, are relatively expensive surface-mount components and consume valuable board surface area. Moreover, to form microwave integrated circuits has been a problem since such devices often require ground planes within which the device is formed.
Current practice is based on combining various RF, microwave, analog, and digital functional blocks (as discrete components) on a low cost flexible printed circuit board, generally of a plastic material. This board may have more than one layer but the buried layers are generally used only as DC or digital interconnects. No RF or microwave components are buried.
The need to reduce the size and weight of these units has led developers to look at low cost multi-layer integration schemes. Reliability, low cost, small size, light weight and performance are main issues in the development of such a technology.